1. Field of the Invention
The present invention relates to a system for electrostatically deflecting a charged-particle beam by means of a pattern yoke. More particularly, the invention is concerned with a charged-particle beam tube which does not cause any deterioration of the beam characteristic even when the deflection sensitivities of charged-particles in two orthogonal directions are different from each other, as well as a method of driving such a tube.
In general, in an electrostatic deflection camera tube, deflection of an electron beam is effected by an electric field produced by a pattern yoke formed on the inner surface of the tube wall. This arrangement enables any beam deflecting coil assembly to be eliminated, thus contributing to reduction in the size, weight and power consumption of video cameras. Electrostatic deflection camera tubes are broadly sorted into two types: namely, a magnetic focusing and electrostatic deflection (MS) type and an electrostatic focusing and electrostatic deflection (SS) type. MS camera tubes are disclosed, for example, in the specification of the U.S. Pat. No. 4,205,253, Japanese Patent Examined Publication No. 46-12213, Japanese Patent Examined Publication No. 57-31257, the specification of the U.S. Pat. No. 4,663,560, and Japanese Patent Unexamined Publication No. 62-206750. On the other hand, SS camera tubes are shown, for example, in the specification of the U.S. Pat. No. 4,658,182, Japanese Patent Unexamined Publication No. 61-7544 and the specification of the U.S. Pat. No. 4,792,721.
In these electrostatic deflection camera tubes, the pattern yoke is composed of a horizontal pair and a vertical pair of deflection electrodes having widths equal to each other in the circumferential direction of the camera tube so as to provide equal deflection sensitivities both in the horizontal and vertical directions. In general, however, the picture screen of a television has a rectangular form with greater dimension in the horizontal direction than in the vertical direction. In particular, the ratio of the horizontal length to the vertical length of the picture screen is as large as 16:9 in the case of modern high-definition television systems. Therefore, when the pattern yoke composed of a horizontal pair and a vertical pair of electrodes of equal circumferential widths is used, the voltage required for deflection of a beam in the horizontal direction (referred to as a horizontal deflection voltage) is higher than the voltage used for the deflection of a beam in the vertical direction (referred to as a vertical deflection voltage), and as a result the scale of the driving circuit is increased undesirably. In order to obviate this problem, it has been proposed to increase the circumferential width of horizontal deflection electrodes as compared with that of vertical deflection electrodes, as disclosed in, for example, Japanese Patent Unexamined Publication No. 62-80943 and No. 63-310542. With this arrangement, it is possible to increase the deflection sensitivity in the horizontal direction, thus enabling a reduction in the total deflection voltage inclusive of both the horizontal deflection and the vertical deflection.
However, it has been found by the applicants that the electrostatic deflection camera tube having such an arrangement of deflection electrodes suffers from the following problems. Namely, the portions of the glass wall of the tube corresponding to the gap between adjacent deflection electrodes, i.e., the portions which are exposed without being covered by the deflection electrodes, are electrostatically charged to produce an electric field called as a astigmatic electric field which acts to cause an astigmatic aberration of the electron beam. The astigmatic electric field also causes drawbacks such as a reduction in the resolution of the image and distortion of the pattern. The charging of the exposed portions of the camera tube wall glass has been encountered also with the case of a conventional camera tube in which the vertical and horizontal deflection electrodes have equal circumferential widths. In these conventional camera tubes, however, no astigmatic electric field is formed because of the symmetry of the horizontal and vertical deflection electrodes, and accordingly, no attention has been paid to the charging on the wall gaps.